The Amateur Amateur: Choking on RF

By Gary Hoffman, KB0H
Contributing Editor
September 14, 2001

The intrepid newcomer encounters common mode problems in a mobile environment.

My automobile is my ham radio laboratory. It was not a conscious decision. That's just where I spend the most time trying to fix things. My wife Nancy and I have similar cars, each equipped with the same model of dual-band Amateur Radio transceiver. Between the two of them, practically everything that can go wrong has gone wrong.

There are several attitudes you can take when dealing with problems. One is to curse, complain, and make everyone around you miserable ("Dag-blasted con-sarn wretched son-of-a-gun is broken again!"). Another approach is to try to learn something ("Okay, why is smoke pouring out of it?"). Frankly, I do both. I burn up the adrenaline cursing, then when I calm down I try the logical approach. I get a great deal of satisfaction if I can actually figure out a problem and then fix it. (And later I write a column about it. It's a win-win situation for me.)

The problem that plagued me the most with our mobile radios was interference. (Real hams call it RFI but I'll stick with calling it "interference.") I couldn't blame external sources since this was self-interference, or more properly, "auto-interference." Let me explain.

My mobile radio was actually two radios built into one chassis. One was for 2 meters, and the other was for 70 cm. They shared a single antenna, a single power supply, a single control mechanism and display, and a single microphone. They did, however, have individual speakers. It was possible to hear conversations on both bands simultaneously, or to hear a conversation on one band while talking on the other.

The problem was that sometimes when I transmitted on one band, I would hear an ear-splitting BRRAAAAAAAAAAPPPPPP! on the other band. It sounded like someone dragging a phonograph needle across a record. It didn't happen every time I transmitted, so I ignored it for as long as I could. Eventually it started to drive me crazy, and it would often happen when I was in the middle of an important conversation.

My first thought was that the dag-blasted con-sarn wretched son-of-a-gun radio was busted, but it happened both on my radio and on Nancy's radio. My second thought was that it was weather related, since I noticed it most often when I was trying to report poor road conditions, but after a few BRRAAAAAAAAAAPPPPPP!s on sunny days, I changed my mind. The third thing that occurred to me was that neither radio had exhibited the problem until I'd switched from glass-mount antennas to magnetic-mount antennas. Was something wrong with the new antennas? Hmmmmm. I thought hard, trying to remember. I'd switched antennas because the glass-mount ones were terribly inefficient.

Time for a Test

toroids in trunk

The chokes on the power cable (left) and speaker cables (right).

The glass-mount antennas were still in place, just not connected to anything. I tried to recreate the problem, but as you all know, appliances never fail when the repairman is there. Eventually, though, I was able to produce a mild buzz while transmitting. I switched to the glass-mount antenna, transmitted, and heard no buzz on the other band, but there was a subtle hiss. My analysis was that the problem was present with both of antennas, but much more evident when I used the magnetic-mount antenna. Ha! Problem solved! All I had to do was move the magnetic-mount antenna from the trunk to the roof!

That didn't help, of course. The problem wasn't the proximity of the antenna to the transceiver, it was merely the efficiency of the antenna--the better the signal, the greater the interference. All right. I hadn't fixed the problem, but I'd sure learned a lot. I knew that the radios had gizmos called "diplexers" that were supposed to prevent a signal transmitted from one band of the radio from getting into the other band. So, I assumed that the interference wasn't getting in through the antenna cable. How about grounding? Manuals always say that grounding is important. I thought the transceiver was well grounded, but made absolutely sure by connecting it with a thick, ugly metal braid. My voltmeter confirmed a good connection, but that didn't solve the problem.

I stared at the spaghetti-like mass of wires in the trunk. Then I had an epiphany (translation: the light bulb over my head lit). Could one or more of these wires be acting like an antenna, picking up my transmissions and feeding them back into the radio? A brief and abortive experiment with tin foil convinced me that I could not effectively shield the wires.

Sitting on my haunches staring into the trunk wasn't helping, so I went inside and sat in my La-Z-Boy instead. The recliner was next to a bookshelf, and I my eyes happened to rest on the ARRL's Operating Manual. Idly I picked it up, thumbed to the index in the back and looked up "interference." Moments later I jerked upright and re-read the section I'd just perused. The very problem I was having was described in detail, and there was a solution! It was called a common mode choke!

toroid on speaker wire

The isolated speaker cable choke.

explanation of chokes

This is a "Common Mode Chokes for Dummies" (for amateur amateurs?) diagram.

All this sounded familiar. Right. I remembered. It was one of the questions on a license exam I had taken some years earlier--common mode chokes, 43 mix ferrite toroids, stuff like that. I closed my eyes and tried to visualize it. Inductors tend to allow lower-frequency signals to pass but block higher-frequency signals. So, if I put inductors on the speaker wires, the audio, being low frequency, should go through unharmed. Any stray radio signals, being much higher in frequency, should be blocked.

I will tell you right now that I am not an electrical engineer. If you talk about microhenrys or picofarads I look like a deer caught in headlights. But in this case I didn't have to calculate anything and barely had to build anything. It was all in the books (The ARRL Operating Manual and The ARRL RFI Book). I bought some 43 mix ferrite toroids (easier than you might think). They were semi-metallic doughnuts, about an inch and a half across. I ran a speaker wire through a doughnut about 15 times (like I was trying to wrap up the doughnut with the wire), and voila! I had an inductor! More specifically, I had a "common mode choke" designed to keep out evil interference.

And did it work? You betcha! Once I'd "choked" both speaker wires the BRRAAAAAAAAAAPPPPPP! diminished to a mere hiss. Later I was able to eliminate even the hiss by putting a choke on the transceiver's power cable. I still receive some auto-interference when I drive through one particular part of town, but hey! That just means I have more to learn.

Editor's note: ARRL member Gary Hoffman, KB0H, lives in Florissant, Missouri. He's been a ham since 1995. Hoffman says his column's name-- "The Amateur Amateur"--suggests the explorations of a rank amateur, not those of an experienced or knowledgeable ham. His wife, Nancy, is N0NJ. Hoffman has a ham-related Web page. Readers may contact the author via e-mail, [email protected].

© 2001 American Radio Relay League


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